This invention relates to improvements in traffic radar devices for law enforcement and related applications in which the speed of a target vehicle is detected from a stationary or moving transmitter location and, in particular, to a method and apparatus for processing Doppler return information by analysis of the entire frequency spectrum in order to improve target identification and minimize interference and unwanted harmonics, and for recognizing the frequency of a selected radar antenna and automatically configuring the system for correct Doppler processing.
Traditional analog traffic radar employs an array of tuneable lowpass, bandpass and notch filters, phase-locked loops and automatic gain control circuitry to determine patrol vehicle and traffic target vehicle speeds. A microwave frequency transmitter sends a low power single frequency carrier through an antenna towards moving vehicle targets. The microwave signal is reflected off the targets and a portion of the reflected signal is received, mixed and amplified by the receiver section of the antenna. This yields an audio frequency sine wave whose frequency (Doppler frequency) is directly proportional to the speed of a target vehicle.
The Doppler frequency is also governed by the cosine effect. If the vehicle is moving directly toward or away from the antenna, resulting in a reflection angle of zero degrees, the return sine wave frequency is directly proportional to the true speed of the vehicle, i.e., the return frequency is equal to vehicle speed times a frequency constant, multiplied by the cosine of the angle between the antenna and the vehicle. Therefore, if the vehicle's direction is at an angle other than zero degrees, the cosine effect reduces the return frequency by the cosine of the angle between the antenna and the vehicle, with the extreme case of a vehicle traveling at a 90 degree angle resulting in a return frequency of zero Hertz.
In stationary mode, where the patrol vehicle is parked, the analog radar bases all target vehicle speed processing on the strongest return signal. In moving mode, where the patrol vehicle is in motion, the assumption is made that the strongest return signal is that of the patrol car and the next strongest signal is that resulting from a target vehicle. Strong forms of interference or signal return instances that violate these criteria render the displayed speeds invalid, requiring that the officer operating the radar recognize the error and ignore the displayed speeds. In these cases as well as all of the other aspects of the operation of a traffic radar, any opportunity for error by the operating officer is to be avoided in order to maximize the credibility of the evidence obtained.